The present invention relates to an impedance matching balun device for coupling an unbalanced transmission line or circuit to either a balanced transmission line or a balanced antenna. A balun, or balanced-to-unbalanced circuit, performs the function of coupling an unbalanced source to a balanced load by providing at the load terminals two signals of approximately equal amplitude and 180.degree. phase difference. Other generally desirable electrical properties are a good input impedance match, low dissipation loss, and flat amplitude and group delay characteristics. The useful frequency bandwidth of a balun is that over which all the above performance factors are satisfied to a specified degree. Implementation of this device may be carried out by a number of well-known methods. Such basic forms are given in Sec. 43-6 of the text Antenna Engineering Handbook, Richard C. Johnson and Henry Jasik, editors, McGraw-Hill, 1984 (Second Edition).
For application in UHF television reception in the frequency range 470-806 MHz, a balun is used to connect a 300-ohm balanced antenna to a 75-ohm unbalanced coaxial cable or amplifier. It is also used to connect 75-ohm cable to the 300-ohm balanced input of the television receiver. The most usual form in present use is the bifilar, transmission line type wound on a ferrite core. These devices at best introduce a loss of 1-2 dB and a mismatch in the 1.5 to 2.0 VSWR (voltage standing-wave ratio) range. In some cases the performance is much worse owing to the difficulty and inconsistent methods of manufacture, wiring, and parts placement. These component deficiencies tend to degrade the receiving system performance and, hence, lessen the overall quality of the UHF television service.
Another known form is disclosed in U.S. Pat. No. 4,160,210 to Molinari. Here a half-wavelength folded microstrip transmission line provides the necessary 180.degree. phase differential at the balanced output. However, as the signal frequency departs from the center design frequency, a phase error results. For the UHF television band, this error is substantial at the band edges, amounting to .+-.47.4.degree..
Another recent example of a balun is shown in U.S. Pat. No. 4,725,792 to Lampe, Jr. The phase error of a half-wavelength line is compensated by a network comprising quarter-wavelength line sections. The bandwidth given for a 4:1 impedance ratio, however, is 20% or somewhat less than required for UHF television. In addition, the size of the circuit becomes rather large and bulky at UHF frequencies in this and other examples which require the use of quarter-wavelength or greater transmission line sections.